The development of the plastic arts has resulted in the widespread use of plastics for a variety of products and applications. A notable example is in the packaging industry where plastic containers are commonly used to package virtually thousands of products, e.g., foods, beverages, detergents, chemicals, and motor oils. These containers can be made from a wide variety of thermoplastic materials such as polypropylene, polyethylene, or polyvinyl chloride. A common method of making thermoplastic containers from such materials is by blow molding, which typically includes the steps of forming a hollow tube of molten thermoplastic material (a "parison"), placing the parison inside a blow mold, expanding the parison by either injecting a pressurized gas into the parison or by creating a vacuum around the parison, maintaining the parison in such an expanded state while it cools and solidifies, and removing the finished container from the mold. U.S. Pat. No. 2,597,558, which issued on May 20, 1952 to Bailey, is representative of such a prior art blow molding method.
Where plastics are used to package goods, it is ordinarily desirable to label the container to provide information about the container and its contents such as brand name, size, composition, source, directions for use, and proper warnings. In addition, a well designed and colored label can significantly enhance the container's appearance and attractiveness. Prior art labeling techniques generally consist of either gluing a printed label to the finished container or directly printing the message on the container's outer surface with a suitable ink or paint. Unfortunately, these prior art methods are relatively expensive because of the additional handling steps required after the container has been molded.
In other prior art labeling techniques, the label is positioned and held on the inner surface of the blow mold cavity prior to blowing the container. This general type of labeling is commonly referred to as "in-mold labeling". One system of this type is shown, for example, in U.S. Pat. No. 3,267,186, which issued to Battenfeld on Aug. 16, 1966. In the method disclosed, a stamp having a series of vacuum passages connected to a vacuum source is used to cut a label from a preprinted strip. Once the label has been cut, the vacuum is activated which holds the label against the stamp's face. The stamp and vacuum-held label are then inserted and held within a complementary-shaped opening in the mold's sidewall such that the stamp becomes an integral part of the mold. When a parison is placed within the mold and injected with a pressurized gas, the parison expands and ultimately comes into adhesive contact with the label. After the parison has fully expanded and solidified, the vacuum to the stamp is shut off and the label adheres to the finished container.
Another prior art in-mold labeling method is shown in U.S. Pat. No. 3,072,969, which issued to Du Bois on Jan. 15, 1963. In the method disclosed, a label is placed on the interior surface of a blow mold in an area having a plurality of spaced air passages that are connected to a vacuum source. When activated, the vacuum holds the label in place while the thermoplastic parison is injected with a pressurized gas. When the expanding parison comes into contact with the label, the vacuum holding the label in place is shut off and the label adheres to the finished container.
While the above-described in-mold labeling methods achieve the basic objective of labeling a blow-molded container, they all share the major disadvantage of requiring a relatively complicated, difficult to make, and expensive blow mold. Furthermore, in the situation where a single parison station serves multiple blow molds, each mold requires the above described complex and expensive vacuum system. Accordingly, it is a primary objective of the present invention to provide an improved in-mold labeling method for blow molded, thermoplastic articles that uses a standard and inexpensive mold.
Another object of the present invention is to reduce the complexity and expense of an in-mold labeling machine by a factor equal to the ratio of mold cavities to parison stations in the situation where a single parison station serves multiple mold cavities.
Another object of the present invention is to label a blow-molded article during the molding operation, thereby eliminating subsequent labeling and handling operations.
Another object of the present invention is to label a blow-molded article during the molding operation, thereby improving the quality of label adhesion and appearance.